1. Field of the Invention
The present invention relates to an optical amplifying apparatus for collectively amplifying wavelength-division multiplexed signal lights, and more particularly, to an optical amplifying apparatus adapted to reduce the influence of the wavelength dependency of gain.
2. Description of the Related Art
There has been a conventional optical amplifying apparatus adapted to perform direct amplification of light by using an optical fiber doped with rare-earth elements, for example, erbium (Er). In the case of this conventional optical amplifying apparatus using a rare-earth-doped optical fiber, the gain thereof has dependence on the wavelength of light. Thus, when wavelength-division multiplexed signal lights obtained by the wavelength-division multiplexing of a plurality of signal lights of plural wavelengths are collectively amplified, there occurs a problem that the gain deviation among gains respectively corresponding to the wavelengths. It is known that the gain tilt among plural wavelengths changes in an optical amplifying apparatus according to the gain of the apparatus. To suppress a change in the gain tilt, there has been proposed, for instance, a conventional system for performing an automatic gain control (AGC) operation so as to control the gain of the optical amplifying apparatus to be constant.
Such a conventional optical amplifying apparatus is, for instance, an optical amplifying apparatus for wavelength-division multiplexing described in Japanese Unexamined Patent Publication No. 8-248455 which is a prior application filed by the present applicant. This conventional optical amplifying apparatus for wavelength-division multiplexing is the one in which two optical amplifiers which have undergone AGC control are connected in cascade, to thereby offset the wavelength-dependency of the gain of the mutual optical amplifiers. Moreover, for example, in Japanese Unexamined Patent Publication No. 9-219696, there has been proposed an optical amplifying apparatus having two-stage constitution by means of two optical amplifiers which have undergone AGC control.
However, in each of the aforementioned conventional optical amplifying apparatuses, when input light power is increased, excitation power supplied to the optical amplifying apparatus reaches a limit, so that the conventional optical amplifying apparatus saturates. Consequently, each of these conventional optical amplifying apparatuses cannot perform a normal AGC operation. In such a case, the gain tilt among the wavelengths of output light is increased owing to the wavelength dependency of the gain of the optical amplifying apparatus. This results in occurrence of a problem that a sufficient input dynamic range for the optical amplifying apparatus cannot be ensured.
For example, a two-stage optical amplifying apparatus that employs optical amplifiers 100 and 200 which are provided with AGC circuits 101 and 201, respectively, as illustrated in FIG. 8. Incidentally, in this case, it is supposed that a variable optical attenuator 300 and a dispersion compensation fiber (DCF) 400 are provided between the optical amplifier 100 provided at a pre-stage and the optical amplifier 200 provided at a post-stage, and that the variable optical attenuator 300 is provided with an automatic level control (ALC) circuit 301 for keeping a level of output signal light OUT at a constant level. FIG. 9 is a diagram showing the level of signal light changing when passing through components of this optical amplifying apparatus.
In FIG. 9, reference characters S1 and S2 designate the saturation level of output light power of the optical amplifier 100 and the saturation level of output light power of the optical amplifier 200, respectively. In this case, an attenuation quantity to be attenuated by the variable optical attenuator 300 is controlled by the ALC circuit 301 in such a manner that the output light power of the optical amplifier 200 becomes constant in the vicinity of the saturation level S2. The gain of the optical amplifier 100 is controlled by the AGC circuit 101 to be constant. As a result, the slope of the signal light level between the input to and the output of the optical amplifier 100 is constant even when the level of the input signal light IN varies in a range of the level from Pi(MIN) to Pi(MAX). Thus, when the maximum level Pi(MAX) of the input signal light IN increases as indicated by a dashed line in FIG. 9, the output light of the optical amplifier 100 saturates. Consequently, to prevent the aforementioned gain tilt between the wavelengths from increasing, only the narrow input dynamic range as indicated by a solid line in FIG. 9 can be ensured.